They run out of places to spawn. You'll have to increase the map size.Yea, I know. It's just that personally I kinda prefer them to not always occupy the poles. Some amount is okay but most of the times many of them are always there. Even if you increases the number of resources or land tiles or even reducing the number of civs, significant number of city states always occupy the poles rather than taking other part of the map rich in resources and also quite far from other potential civilization development region